Insulator shielded cathode



April 28, 1970 c. M. BANAS ETAL 3,509,410

INSULATOR SHIELDED CATHODE Original Filed Nov. 3, 1965 'r Y a I l I Q CDi AHMP/ F, I T s l a l 5. t 0% I H I HIGH DvgLTAGE POWER SUPPLYINVENTORS CONRAD M. BANAS CLYDE 0. BROWN ATTORN EY United States Patent3,509,410 INSULATOR SHIELDED CATHODE Conrad M. Banas, Manchester, andClyde 0. Brown,

Newington, Coun., assignors to United Aircraft Corporation, EastHartford, Conn., a corporation of Delaware Continuation of applicationSer. No. 506,237, Nov. 3, 1965. This application Sept. 11, 1967, Ser.No. 667,010 Int. Cl. H01j 61/04, 61/10 U.S. Cl. 313207 6 Claims ABSTRACTOF THE DISCLOSURE A cathode operating in a glow discharge for producingan electron beam is provided with an insulator shield for selectivelysuppressing the flow of electrons from those areas covered by theshield.

This application is a continuation of application Ser. No. 506,237,filed Nov. 3, 1965.

Conventional methods of producing electron beams involve liberation ofelectrons from the surface of a heated cathode by thermionic emission.Recently, focussed electron beams have been produced from a hollowcathode operated in a glow discharge mode. The presence of a plasma nearthe external surfaces of the cathode has been found to be detrimental toefiicient operation since most of the energy supplied to maintain thisplasma is dissipated at the cathode. Substantial improvement in the beampower efficiency of such cathodes can be obtained by shielding theexternal cathode surfaces in such a manner as to eliminate the glowdischarge from these regions.

Shields for these cathode discharge devices have been made of conductivematerials and found to improve beam power efficiency. However, it hasbeen noted that such shielding significantly reduces the ambientpressure range for stable electron beam mode operation of the discharge.Further, this shielding technique has been found to lead to sporadicarcing between cathode and shield. These disadvantages may be obviatedand the electron beam mode operation of a glow discharge cathode may besubstantially improved by shielding it with an insulator material.

It is accordingly an object of this invention to provide an insulatorshield for a cathode operating in the glow discharge mode.

This and other objects of this invention will become more readilyapparent upon a review of the description and the accompanying figure.

In this invention a cathode operating in the glow discharge mode issurrounded by an insulator shield which is spaced from the cathodesurfaces at a predetermined distance to essentially eliminate arcformation between the cathode and the shield, and to exclude thedischarge plasma from the region adjacent to the external surfaces ofthe cathode.

In the figure, a conventional hollow cathode 1 is shown which isenclosed in a chamber. This cathode is well known and may be of the typedisclosed in U.S. Patent 3,381,157. The hollow cathode assembly in thefigure is made up of two cylindrical sections 2, a solid shaft whichserves as the high voltage lead and support, and 1 the hollow cathode.The hollow cathode 1 may be removably mounted to the solid portion 2 bymeans of a banana plug arrangement or by means of a threaded studattached to the hollow cathode section 1 and extending into the solidsection 2. The solid section 2, which is also conductive, is connectedto a conductive stud 3 either by a threaded arrangement similar to thatused for section 1 or forms a continuous conductive body therewith. Thestud 3 extends through a high voltage bushing 4 and a hermetic seal isformed between the stud 3 and the bushing 4 by brazing at 5.

The cathode 1 is exposed to an evacuated chamber 6 in which is disposedan anode 7 at a suitable axial distance from the cathode 1. The anode 7may also be the workpiece. The cathode 1 is also provided with anaperture 8 which has the proper dimensions necessary to provide thehollow cathode discharge. The vacuum chamber 6 is evacuated to theproper pressure needed to support the glow discharge. An inert gasatmosphere is customarily used to reduce contamination in metal-workingapplications. A high voltage supply 12 biases the cathode negative withrespect to the anode 7.

Surrounding the cathode 1 is an insulator shield 9 which is spaced fromthe cathode 1 by gap 10. The gap 10 is determined as a function of thepressure and voltage operation of the cathode in accordance withwellknown gas discharge scaling relationships. The insulator is alsoshown to terminate a short distance 11 from the face of the cathode toenhance the efiiciency of this device. The gap 10 extends all along thebody of the cathode 1 and support shaft 2.

The insulator shield 9 effectively inhibits the formation of a glowdischarge in the gap 10 which is sized to suppress the formation of theplasma therein. The benefit of a shield formed from an insulatingmaterial arises from a reduction of the high potential gradients in thegap 10 and at the junction between the high voltage lead 3 and insulator9 as compared with those obtainable with conductive shields. Reductionof the potential gradients alleviates high voltage breakdown problems.Consequently, even though the plasma which is essentially at anodepotential and may partially extend within the gap 10, there is a markedreduction in high voltage arcs between the cathode and the shield whenan insulator material is used.

Cathodes of solid wall construction may be effectively used with aninsulating shield. Such cathodes are much simpler and easier toconstruct than screen cathodes and may operate with beam powerefiiciencies in excess of Typical refractory materials from which theshield 9 may be made are alumina, zirconia, boron nitride, or beryllia.These shield materials are specified on the basis of their highdielectric strength at elevated temperatures. Due to radiant heating bythe cathode 1, portions of the shield may approach temperatures of theorder of 1500 K. under high power level conditions. For low beam powerapplications, a nonrefractory insulator shield could be used withadvantage. 7

It is to be understood that the invention is not limited to the specificembodiment herein illustrated and described but may be used in otherways without departture from its spirit as defined by the followingclaims.

We claim:

1. In an electron beam apparatus,

an enclosure containing a relatively low pressure ion- 1 izable gaseousmedium,

a cathode positioned Within said enclosure and comprising a hollowelectrically conductive structure having at least one beam exitaperture,

an anode positioned within said enclosure,

means for applying potentials to said anode and said cathode whereby aplasma may be generated within said cathode to effect a nonthermionicelectron beam mode of operation of said cathode whereby an electron beamissues from the plasma through said beam exit aperture, and

an insulating shield substantially surrounding said cathode and spacedat a short distance therefrom for suppressing any glow dischargeexterior to said cathode.

2. An electron beam apparatus as in claim 1 in which said cathode iscylindrical and in which said aperture is formed in a base of saidcylindrical cathode,

said insulating shield extending fully aboutsaid cathode and terminatinga short distance from the apertured base of said cathode.

3. An electron beam apparatus as in claim 1 and including support meansfor said cathode extending from a wall of said enclosure, saidinsulating shield surrounding said support means.

4. An electron beam apparatus as in claim 1 in which said insulatingshield is spaced from said cathode at a distance which suppressesformation of a plasma therebetween.

5. An electron beam apparatus as in claim 1 in which said insulatorshield comprises a material having a high dielectric strength atelevated temperatures.

UNITED STATES PATENTS 2,112,718 3/1938 Somers 313-205 X 2,238,277 4/1941Miller 313205 2,433,813 12/1947 Hilliard 3l3-219 X 3,242,371 3/1966Sugawara et al. 313219 X 3,320,475 5/1967 Boring 315108 ROBERT SEGAL,Primary Examiner U.S. Cl. X.R. 313205, 339

